I have just done a flight in the 757-200 from YMML to YSSY.

I used LNAV for the climb, and it was a normal climb of between 2000 and 4000+ fpm.
Once I reached my cruise altitude of FL380, I switch to SPD and set it for M.80. It worked fine. First it increased the throttle to reach M.80, and then the thottles move back forth a little bit until it had adjusted to hold M.80.
Then I entered .800 in the FMC (VNAV CRZ) and the result was the same as for using SPD without VNAV.

So the result is that it is working normally for me. I'm not really sure why yours doesn't. :(

Mark